1. Field of the Invention
The present invention relates to a reverse conducting semiconductor device in which both an insulated gate bipolar transistor (IGBT) element region and a diode element region are provided in the same semiconductor substrate.
2. Description of the Related Art
Reverse conducting semiconductor devices exist in which both a region where an insulated gate bipolar transistor (IGBT) is formed (IGBT element region) and a region where an free wheel diode (FWD) is formed (diode element region) exist in the same semiconductor substrate. Because two kinds of elements exist in reverse conducting semiconductor devices, it is difficult to form an optimal structure for both of the elements in the same semiconductor substrate. Japanese Patent Application Publication No. 2005-317751 suggests that the recovery loss when the diode is shifted from a conductive state to a non-conductive state is greater in a reverse conducting semiconductor device than in IGBT and the diode formed in separate substrates. To overcome this problem, a semiconductor device 100 (see FIG. 14 attached to the present application) described in JP-A-2005-317751 includes a low lifetime layer 161. Below, the structure and operation of the semiconductor device 100 will be described.
The semiconductor device 100 includes an n−-type layer 160 that extends across both an IGBT element region J101 and a diode element region J102. The n−-type layer 160 serves as a drift layer in the IGBT element region J101. In addition, the n−-type layer 160 serves as an n−-type cathode layer (high-resistance layer) in the diode element region J102. In this specification, the drift layer and the high-resistance layer will be generically referred to as drift layer. Hereinafter, the n−-type layer 160 will be referred to as drift layer 160. The low lifetime layer 161 is formed at the intermediate depth of the n−-type layer 160. The low lifetime layer 161 is formed by bombarding a lifetime killer (such as helium) from the front surface 102a of a semiconductor substrate 102. The low lifetime layer 161 extends across the drift layer 160 in the IGBT element region J101 and the drift layer 160 in the diode element region J102. In the low lifetime layer 161, the lifetime of minority carriers (holes) is short.
When a voltage higher than that applied to a back-surface electrode 103 is applied to a front-surface electrode 101 of the semiconductor device 100, holes flow out from a high-concentration p-type region 122 that is formed to face the front surface 102a of the semiconductor substrate 102. The holes are injected into the drift layer 160 via a low-concentration p-type layer 130. Also, electrons flow out from a cathode region 170 that is formed to face a back surface 102b of the diode element region J102, and are injected into the drift layer 160. Electric current flows between the anode and the cathode (between the high-concentration p-type region 122 and the cathode region 170), so that the diode element region J102 is shifted into a conductive state. When the voltage to the front-surface electrode 101 falls below the voltage to the back-surface electrode 103, holes are no longer injected from the high-concentration p-type region 122 into the drift layer 160. The diode element region J102 is thus shifted into a non-conductive state.
As the diode element region J102 is shifted from a conductive state to a non-conductive state, a phenomenon occurs in which the holes injected into the drift layer 160 return to the low-concentration p-type layer 130. As a result, recovery current flows in the diode element region J102 in a direction opposite from that when conductive. When the recovery current flows, loss occurs, and the diode element region J102 generates heat. The semiconductor device 100 includes the low lifetime layer 161. Some of the holes that return to the low-concentration p-type layer 130 at the time of a recovery operation are lost in the low lifetime region 161. The provision of the low lifetime layer 161 makes it possible to reduce the recovery current in the diode element region J102, thus enabling reduction in recovery loss in the diode element region J102.
When the reverse conducting semiconductor device described in JP-A-2005-317751 is used, it is possible to reduce recovery loss in the diode element region J102. However, in the IGBT element region J101, this is likely to adversely affect the conductivity modulation phenomenon when the low lifetime layer 161 is in an ON state, thereby increasing the voltage in the IGBT element region J101.